The present invention relates generally to a circuit protection device. In particular, the invention relates to a device for protecting electrical circuits against electrostatic discharge (xe2x80x9cESDxe2x80x9d) events and overcurrent conditions.
It is well known that the resistivity of many conductive materials change with temperature. Resistivity of a positive temperature coefficient (xe2x80x9cPTCxe2x80x9d) material increases as the temperature of the material increases. Many crystalline polymers, made electrically conductive by dispersing conductive fillers therein, exhibit this PTC effect. These polymers generally include polyolefins such as polyethylene, polypropylene and ethylene/propylene copolymers. Certain doped ceramics such as barium titanate also exhibit PTC behavior.
At temperatures below a certain value, i.e., the critical or switching temperature, the PTC material exhibits a relatively low, constant resistivity. However, as the temperature of the PTC material increases beyond this point, the resistivity sharply increases with only a slight increase in temperature.
Electrical devices employing polymer and ceramic material exhibiting PTC behavior have been used as overcurrent protection in electrical circuits. Under normal operating conditions in the electrical circuit, the resistance of the load and the PTC device is such that relatively little current flows through the PTC device. Thus, the temperature of the device due to I2R heating remains below the critical or switching temperature of the PTC device. The device is said to be in an equilibrium state (i.e., the rate at which heat is generated by I2R heating is equal to the rate at which the device is able to lose heat to its surroundings).
If the load is short circuited, the current flowing through the PTC device increases and the temperature of the PTC device (due to I2R heating) rises rapidly to its critical temperature. At this point, a great deal of power is dissipated in the PTC device and the PTC device becomes unstable (i.e., the rate at which the device generates heat is greater than the rate at which the device can lose heat to its surroundings). The power dissipation only occurs for a short period of time (i.e., a fraction of a second), however, because the increased power dissipation will raise the temperature of the PTC device to a value where the resistance of the PTC device has become so high that the current in the circuit is limited to a relatively low value. This new current value is enough to maintain the PTC device at a new, high temperature/high resistance equilibrium point, but will not damage the electrical circuit components. Thus, the PTC device acts as a form of a fuse, reducing the current flow through the short circuit path to a safe, relatively low value when the PTC device is heated to its critical temperature range.
Upon interrupting the current in the circuit, or removing the condition responsible for the short circuit, the PTC device will cool down below its critical temperature to its normal operating, low resistance state. The effect is a resettable, electrical circuit protection device.
Electrical overstress transients (xe2x80x9cEOS transientsxe2x80x9d) produce high electric fields and high peak powers that can render circuits or the highly sensitive electrical components in the circuits, temporarily or permanently non-functional. EOS transients can include transient voltages or current conditions capable of interrupting circuit operation or destroying the circuit outright. EOS transients may arise, for example, from an electromagnetic pulse, an electrostatic discharge, lightning, a build up of static electricity or be induced by the operation of other electronic or electrical components. An EOS transient can rise to its maximum amplitude in subnanosecond to microsecond times and have repeating amplitude peaks.
The peak amplitude of the electrostatic discharge (ESD) transient wave may exceed 25,000 volts with currents of more than 100 amperes. There exist several standards which define the waveform of the EOS transient. These include IEC 1000-4-2, ANSI guidelines on ESD (ANSI C63.16), DO-160, and FAA-20-136. There also exist military standards, such as MIL STD 883 part 3015.
Materials exist for the protection against EOS transients (xe2x80x9cEOS materialsxe2x80x9d), which are designed to rapidly respond (i.e., ideally before the transient wave reaches its peak) to reduce the transmitted voltage to a much lower value and clamp the voltage at the lower value for the duration of the EOS transient. EOS materials are characterized by high electrical resistance values at low or normal operating voltages and currents. In response to an EOS transient, the materials switch essentially instantaneously to a low electrical resistance state. When the EOS threat has been mitigated these materials return to their high resistance state. These materials are capable of repeated switching between the high and low resistance states, allowing circuit protection against multiple EOS events.
EOS materials also recover essentially instantaneously to their original high resistance value upon termination of the EOS transient. For purposes of this application, the high resistance state will be referred to as the xe2x80x9coff-statexe2x80x9d and the low resistance state will be referred to as the xe2x80x9con-state.xe2x80x9d EOS materials can withstand thousands of ESD events and recover to desired off-status after providing protection from each of the individual ESD events.
Circuit components utilizing EOS materials can shunt a portion of the excessive voltage or current due to the EOS transient to ground, thus, protecting the electrical circuit and its components. The major portion of the threat transient, however, is reflected back towards the source of the threat. The reflected wave is either attenuated by the source, radiated away, or re-directed back to the surge protection device which responds with each return pulse until the threat energy is reduced to safe levels.
In certain situations, an EOS voltage may be continuous rather than transient in nature. For example, if an AC power line falls across a telecom line, a continuous abnormally high voltage may be induced into the telecom equipment at both ends of the line. Another example involves the connection of the wrong battery charger or battery eliminator to a piece of portable electronic equipment such as a cell phone. In both of these examples, specific types of overvoltage protectors might be used to protect the equipment from the abnormally high continuous voltage for a finite period of time. It is also common to rely on an overcurrent protector to limit current to the overvoltage protector and associated equipment to protect against self-heating.
It should be appreciated from the foregoing discussion that many electrical and electronic circuits require overcurrent and overvoltage protection. FIG. 1 shows a circuit diagram of a typical circuit incorporating overcurrent and overvoltage protection. Generally, the overcurrent and overvoltage protection is obtained through at least two discrete devices. Each device provides protection for a specific application. For example, a discrete PTC current limiter 10 provides protection during overcurrent situations. In addition, a discrete ESD protection device 12 provides protection during ESD events or EOS transients. The two discrete devices are interconnected through printed circuit board traces. As a result, valuable space of the printed circuit board is utilized by the footprint of each discrete component.
Printed circuit board designers are always looking for ways to reduce the footprint of components in an effort to reduce the circuit board space needed. Thus, there is a need to reduce the size of integrated overcurrent and overvoltage protection devices.
A number of other disadvantages also occur when circuits have discrete electrical protection devices. Electrical coordination problems arise with the discrete devices making it difficult to assure coordination between the voltage suppressor protecting at overvoltage conditions and the thermal protector protecting during overcurrent conditions. Coordination between devices is important to eliminate damage to a circuit and its components in any overcurrent or overvoltage condition. One reason that makes coordination between the discrete devices difficult is because discrete devices are often times made by different manufacturers. As such, the burden is placed on the circuit board designer to coordinate the discrete devices together in order to obtain the proper overcurrent and overvoltage protection of the circuit and its components. Determining proper electrical coordination between the devices is not a simple procedure. It takes time to evaluate criteria of each device (e.g., time versus current curves, voltage versus current curves, etc.) to make sure that protection against excessive voltages and currents will result. In addition, specification tolerances for the discrete devices of different manufacturers may vary resulting in poor coordination between the discrete devices even when a proper evaluation is conducted by the designer.
Another concern exists with the existing discrete devices is that the discrete devices may not be fast enough during overvoltage conditions for certain applications. To this extent, sensitive circuitry can easily be damaged.
A need therefore exists for an improved means for providing protection against ESD events and excessive currents.
The present invention provides an improved circuit protection device having a PTC current limiter and an ESD protection material within a single package. The improved circuit device achieves overcurrent and overvoltage protection. At the same time, the device of the present invention provides accelerated PTC switching during an overvoltage condition due to a high degree of thermal coupling. Moreover, the device of the present invention assures coordination between a voltage suppressor and a PTC element.
To this end, in an embodiment of the present invention, an electrical circuit protection device is provided. The device includes an overcurrent protection portion having a surface. A voltage variable material is disposed on the surface. An overcurrent termination is provided on the device, which electrically communicates with the overcurrent protection portion. An overvoltage termination is provided on the device, which electrically communicates with the voltage variable material.
In an embodiment, the overcurrent protection portion includes a PTC element.
In an embodiment, the overcurrent protection portion includes a first substrate and a second substrate, each substrate having an electrode disposed thereon, and a PTC element positioned between the first and second substrates and electrically connected to the electrodes.
In an embodiment, voltage variable material is disposed onto the outer surfaces of both substrates.
In an embodiment, the overcurrent termination electrically communicates with the overcurrent protection portion and the voltage variable material.
In an embodiment, the device includes a plurality of overvoltage terminations.
In an embodiment, the device includes at least one gap on the surface between two terminations, wherein each gap is covered by the voltage variable material.
In an embodiment, the device includes at least one overcurrent termination, at least one overvoltage termination and at least one a common termination.
In an embodiment, the device includes a spark gap on the surface between the overcurrent termination and the common termination or between the overvoltage termination and the common termination, wherein the gap is covered by the voltage variable material.
In an embodiment, the device includes a plurality of overvoltage terminations that are configured to electrically communicate with a plurality of data lines disposed on a printed circuit board.
In an embodiment, the overvoltage and overcurrent terminations are configured to electrically communicate on a printed circuit board with a Universal Serial Bus port or an Institute of Electrical and Electronics Engineers (xe2x80x9cIEEExe2x80x9d) 1394 port.
In an embodiment, the device is provided in an 1812, 2029 or 3425 package size.
In an embodiment, the voltage variable material includes a matrix of conductive and semiconductive particles.
In an embodiment, the voltage variable material includes irregularly shaped conductor particles having insulative oxide surface coatings that are bound in a matrix.
In an embodiment, the voltage variable material includes a mixture of conductive and semiconductive particles having surfaces coated with an insulative oxide film, the conductive and semiconductive particles being bound together in an insulative binder.
In an embodiment, the voltage variable material includes conductive and semiconductive particles that are coated with an insulative oxide film and that are bound together in point contact with one another.
In another embodiment of the present invention, an electrical circuit protection device is provided. The device includes an overcurrent protection portion having a first surface. A voltage variable material is disposed on the first surface. At least one termination for each of the overcurrent protection portion, the voltage variable material and a common connection is provided on the device. A gap on the first surface between two terminations is provided, wherein the gap is covered by the voltage variable material.
In an embodiment, the overcurrent protection portion has a second surface and voltage variable material disposed on the second surface.
In an embodiment, the device includes a gap between two terminations on the second surface, wherein the gap is covered by the voltage variable material.
In a further embodiment of the present invention, an electrical circuit protection device is provided. The device includes a first substrate having top and bottom surfaces and an electrode disposed on its top surface. A second substrate is provided having top and bottom surfaces and an electrode is disposed on its bottom surface. A PTC element is positioned between the first and second substrates and electrically connects to the electrodes. Voltage variable material is disposed on the top surface of the second substrate.
In an embodiment, the device includes voltage variable material disposed on the bottom surface of the first substrate.
Accordingly, it is an advantage of the present invention to provide a discrete overvoltage and overcurrent protection device.
A further advantage of the present invention is to provide an improved device that reduces the footprint requirements on a printed circuit board and provides both overcurrent and overvoltage protection.
A still further advantage of the present invention is to provide an improved device having improved PTC switching characteristics during a continuous overvoltage condition.
Another advantage of the present invention is to provide an improved device that assures electrical coordination between the overvoltage protection and the overcurrent protection.
Yet another advantage of the present invention is to provide an improved device that readily connects to the transmission lines and other lines of known data transfer interfaces.
Additional features and advantages of the present invention will be described in and are apparent from the detailed description of the presently preferred embodiments and the figures.